need help about IR

[airguy2012]

From very first minutes when align, no position detected until after several minutes passed then position comes on on display.
Does anybody know how a/c knows its initial position at specific geographic location?
Thank you for ur help!

[darkroomsource]

The same way your SATnav (or GPS) in your car figures out where you are. And for the same reason it doesn't know right away.

[T.R Haychemu]

Darkroomsource, how does Inertial Reference alignment compare to a GPS aquiring its position then?

An IRS system itself, with no external input (FMGC/GPS) can only work out lattitude and direction by measuring earth rate.

To quote the D/O for the alignment phase;


The alignment is the principal initialization mode for the IR. Its primary function is to initialize the attitude, velocity and position integration functions implemented in the navigation mode. This mode operates on ground only.

The IR alignment mode is divided into 3 parts:

coarse level processing
The coarse level processing is engaged during the first 30 seconds of the IR alignment mode. This processing estimates the local vertical using the 3 accelerometers and the measured gravity.

During coarse level processing of the alignment mode, the software computes the level coordinate frame, pitch and roll, and associated rates and accelerations.



gyro-compass (or azimuth) processing and level processing
Gyro-compass processing is engaged after the 30 seconds of the IR alignment mode (coarse level complete) and runs for a minimum of 9.5 minutes.

Gyro-compass processing is used to orient body frame to North (using earth rotation detection by gyros).

During this submode of the alignment, an estimated latitude is computed using local vertical component of the earth rotation.

During this submode, the results of the coarse level processing are sharpened to have a better knowledge of the coordinate frame.


position entry data processing
This position initialization can take place in coarse level processing or gyro-compass processing.

The latitude and longitude pair processed for position initialization is received from the same source (FMGEC 1 or FMGEC 2).

As soon as a valid position initialization pair is received, the software performs a BITE test to check if the entered latitude and longitude are within the following limits versus the position recorded at the end of the last power up cycle:

Abs val( lat entered - lat recorded ) less than or equal to 1 deg.
Abs val( long entered - long recorded ) less than or equal to 1 deg. (Ref. 34-18-00)
A second BITE test is performed on the entered latitude when an estimated latitude is available during the gyro-compass processing:
Abs val( cos (lat est) - cos (lat ent) ) less than or equal to 0.01234.
Abs val( sin (lat est) - sin (lat ent) ) less than or equal to 0.01234.


The alignment is completed after 10 minutes if a valid position data has been received and verified by the IR.

[bcgallacher]

Inertial systems have to be told where they are on alignment - lat. and long.positions have to be inserted.The memory in the system will retain the position on shutdown - that is why if you move the aircraft to another parking position it will give a warning when the new present position is inserted.

[darkroomsource]

I didn't mean it had to acquire a satellite signal, I meant it had to acquire the Lat/Lon information.
That is not an instantaneous thing, so it takes time to get it.